Existing spectral-beam combiners that incorporate wavelength-dispersion compensating techniques (such as described in U.S. Pat. No. 7,199,924 and U.S. Pat. No. 7,233,442, which are commonly owned by the assignee of the present invention) use input beams that propagate parallel to one another in a single plane, thus limiting the number of laser beams that can be conveniently combined into a single output beam.
The broad gain bandwidth of conventional fiber-laser systems allows for operation over a wide range of wavelengths, or even tunable operation. For the simplest fiber laser system with cavity mirrors having reflectivity across a broad range of wavelengths, the output wavelength can be very broad and can vary with pump power, fiber length, and/or other parameters. The power that can be generated from fiber lasers and fiber-laser amplifiers can often be limited by nonlinear optical effects in the gain and/or delivery fibers used in the system.
It is desirable to produce high peak and average powers from fiber lasers and amplifiers. Stimulated Brillouin scattering (SBS) and other nonlinear effects such as self-phase modulation (SPM), four-wave mixing (FWM), and stimulated Raman scattering (SRS) are the main effects limiting the output power and pulse energy of a fiber amplifier or laser. To suppress these effects in a fiber amplifier/laser, it is desirable to use a rare-earth-doped (RE-doped) fiber with a large core. The large core provides two benefits: Spreading the light over a larger core decreases the intensity driving the nonlinear processes, and increasing the core/cladding diameter ratio increases pump absorption, enabling the shortening of the fiber to further reduce nonlinearities. When good beam quality is required, however, increasing the core diameter of the fiber requires that the fiber numerical aperture (NA) be decreased, in order that higher-order modes cannot propagate in the fiber. Using relatively large-core, low-NA fibers with mode-filtering techniques has been demonstrated to achieve good beam quality, but there are practical disadvantages to the use of such fibers. Fibers with very low values of NA exhibit large bending losses, even for relatively large-radius bends. With fibers having the lowest NA, the fiber must be kept quite straight, otherwise the optical amplifier and/or laser has very low efficiency as the bending loss becomes too high. Since a typical laser oscillator or amplifier might require on the order of a meter or more of gain fiber, the inability to coil the fiber has precluded compact packaging of the fiber-laser system.
Stimulated Brillouin scattering (SBS) is a well-known phenomenon that can lead to power limitations or even the destruction of a high-power fiber-laser system due to sporadic or unstable feedback, self-lasing, pulse compression and/or signal amplification.
Even when a fiber amplifier or fiber laser is designed to compensate for the above effects, there will be a limit on the maximum power that can be obtained from a single fiber when scaling to larger fiber sizes and/or lengths, pump powers, and the like.
U.S. Pat. No. 6,192,062 to Sanchez-Rubio et al. entitled “Beam combining of diode laser array elements for high brightness and power” and U.S. Pat. No. 6,208,679 to Sanchez-Rubio et al. entitled “High-power multi-wavelength external cavity laser” describe the fundamental techniques of spectral beam combining, and both are incorporated herein by reference.
In some embodiments, the gratings used for spectral-beam combining are “blazed,” i.e., formed with V-grooves having sidewall angles that are asymmetrical with respect to a vector normal to the overall surface of the grating. U.S. Pat. No. 3,728,117 to Heidenhain et al. entitled “Optical Diffraction Grid” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves. U.S. Pat. No. 4,895,790 to Swanson et al. entitled “High-efficiency, multilevel, diffractive optical elements” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves using binary photolithography to create stepped profiles. U.S. Pat. No. 6,097,863, titled “Diffraction Grating with Reduced Polarization Sensitivity” issued Aug. 1, 2000 to Chowdhury (incorporated herein by reference) describes a reflective diffraction grating with reduced polarization sensitivity for dispersing the signals. The Chowdhury grating includes facets that are oriented for reducing efficiency variations within a transmission bandwidth and that are shaped for reducing differences between the diffraction efficiencies in two orthogonal directions of differentiation. U.S. Pat. No. 4,313,648 entitled “Patterned Multi-Layer Structure and Manufacturing Method” issued Feb. 2, 1982 to Yano et al. (incorporated herein by reference) describes a manufacturing method for a patterned (striped) multi-layer article.
U.S. Pat. No. 6,754,006 entitled “Hybrid metallic-dielectric grating” issued Jun. 22, 2004 to Barton et al. and is incorporated herein by reference. This patent describes a diffraction grating having a metallic base layer and layers of dielectric materials of varying refractive index, where a bottom interface of the layers is adherent to the metallic base layer. The dielectric layers are periodically spaced on top of the metallic base layer, leaving the metallic base layer exposed in regions. This grating allows for the polarization-insensitive reflective properties of the base metallic layer to operate in conjunction with the polarization sensitive diffraction properties of the multilayer grating structure to provide near 100% diffraction efficiency over a reasonable wavelength bandwidth, independent of the polarization of the incident beam.
U.S. Pat. No. 6,822,796 to Takada et al. entitled “Diffractive optical element” (incorporated herein by reference) describes a method for making blazed gratings having asymmetric grooves with dielectric coatings. U.S. Pat. No. 6,958,859 to Hoose et al. entitled “Grating device with high diffraction efficiency” (incorporated herein by reference) describes a method for making blazed gratings having dielectric coatings.
U.S. Pat. No. 5,907,436 entitled “Multilayer dielectric diffraction gratings” issued May 25, 1999 to Perry et al., and is incorporated herein by reference. This patent describes the design and fabrication of dielectric grating structures with high diffraction efficiency. The gratings have a multilayer structure of alternating index dielectric materials, with a grating structure on top of the multilayer, and obtain a diffraction grating of adjustable efficiency, and variable optical bandwidth.
U.S. Pat. No. 6,212,310 entitled “High power fiber gain media system achieved through power scaling via multiplexing” issued 3 Apr. 2001 to Waarts et al., and is incorporated herein by reference. This patent describes certain methods of power scaling by multiplexing multiple fiber gain sources with different wavelengths, pulsing or polarization modes of operation is achieved through multiplex combining of the multiple fiber gain sources to provide high power outputs, such as ranging from tens of watts to hundreds of watts, provided on a single mode or multimode fiber. One method described by Waarts et al. is similar to that shown in the present invention shown in FIG. 2A, described below, where a plurality of input laser beams of differing wavelengths are directed at different angles to a diffraction grating, which diffracts the beams into a single output beam, however, this output beam necessarily has a wavelength linewidth-dependent chromatic divergence introduced by the grating. The present invention includes many distinguishing features not in Waarts et al.
In some embodiments, the present invention uses features and/or is combined with features described in commonly owned U.S. Pat. No. 7,199,924, U.S. Pat. No. 7,391,561, and U.S. Pat. No. 7,233,442, and U.S. patent application Ser. Nos. 11/565,619, 11/426,302, 11/688,854 and 11/556,658, each of which is incorporated herein by reference.
When combining many high-power laser beams into a single beam, the last output grating is impinged by the highest power. Even when using highly reflective dielectric gratings, some power is absorbed by the output grating, which causes that grating to increase in temperature and distort. In order to reduce the power density at the output grating (to reduce its temperature and distortion), it is necessary to increase the beam diameter. Previous dual-grating approaches could only combine a limited number of beams due to geometric constraints.
There is a need for improved laser systems, particularly fiber lasers and/or fiber optical amplifiers, wherein the optical outputs from a larger number of optical-fiber amplifiers and/or other lasers are combined into a single beam.